Again, on windows, the directory are here:
C:\Users\YourAccName\Documents\My Games\Runic Games\Torchlight 2\save\XXXXXXXXXXXXXXXXX\
C:\Users\YourAccName\Documents\My Games\Runic Games\Torchlight 2\modsave\XXXXXXXXXXXXXXXXX\

Your character save in unmodded gameplay can be found at "save" folder
Your character save in modded gameplay can be found at "modsave" folder

You are free to transfer (or backup somewhere else) your character save file, that looks like this:
DOUDLEY_{D024F265-9684-403D-8808-C4C14834616B}.SVB

NOTE: The file extension is "SVB" and my character name is "DOUDLEY" in this example (finding your character name, you will know if it is your character)

NOTE2: When a mod page says that you backup your save files, it means that you make a copy of these "SVB" files anywhere away from the save/modsave folder or inside these folders but make sure it won't be recognized by the game (like creating a new folder and put your backups in there).

As "doudley" said:

Copy and Paste your "character name" SVB file from the "save" folder over to the "mod save" folder.

I keep an additional copy safe outside in another folder when I need it.

Very easy! ^_^

ok cool, thanks guys!